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THE
FEBRUARY 16, 1924
MUSIC TRADE
REVIEW
15
The Various Types of Drying Kilns
Second of a Series of Articles by Thomas D. Perry, Vice-President and Manager of the Grand Rapids Veneer
Works, Grand Rapids, Mich., on the Principles of Lumber Drying, Operating Tests, Different
Types of Kilns in Use and Their Accessory Equipment
IANO makers are one of the oldest groups
of highly skilled wood workers and have
long appreciated the necessity for well-
seasoned lumber that would be light in weight,
and would not change shape nor distort under
various conditions of assembling or fabrication,
nor under diverse climatic conditions after com-
pletion and possession by customers.
Early Kiln Experiences
The original dry kiln was the great out of doors,
with the sun shining to produce temperature,
the wind blowing to give ventilation in the piles
of lumber, and the rain and fog to prevent too
rapid surface drying. For reducing lumber to
final manufacturing dryness nature was too un-
certain and too slow, so that a limited amount
of lumber was selected for the next few pianos
and set edgewise on racks above the work
benches, where it was subject to the final dry-
ing influence of the warm air in the top of
the room. Before long the racks came to be
located over the stove or steam boiler, if the
plant was large enough to have one.
Artificial Dryers
Gradually artificial dryers came into use, and
at first were vertical or horizontal pipe racks
or grids arranged so that the boards to be
dried were placed between layers of heated
steam coils. This particular type is some-
times seen even to-day, with steam or hydraulic
jacks to separate the coil-grids or platens and
permit the insertion of lumber, but the device
produces lumber excessively casehardened, with
overdried surfaces, and likely to be checked in-
ternally or externally.
Instead of heated steam grids or platens some
manufacturers used blowers or fans to circulate
the dry, warm air more actively among the
boards during final drying, and sometimes whole
rooms were devoted to such fan drying.
Obsolete Drying Equipment
Many present-day piano factories are handi-
capped with an inheritance of obsolete drying
equipment that has outlived its usefulness. It
is cheaper to discard an inefficient piece of dry-
ing apparatus than to maintain and operate it.
But home-made dry kilns have had their day,
and in their place have come several different
types of modern kilns, all of which have en-
thusiastic advocates and ardent supporters.
Kiln design and construction has become a spe-
cial branch of manufacturing and has its own
recognized corps of trained engineers and
experts.
Three Types of Kilns
The three main divisions into which lumber
kilns may be grouped, as illustrated, are:
P
utilizing the fundamental laws of physics to ob-
tain internal circulation.
Cut No. 6
A =
heating unit; B =
condensing unit; E = lumber
2. Condenser—Generally of the gravity-recir-
culating type in which the air passes over mois-
ture-removal or condensing units, one in each
interchange.
fresh-air inlet; D and D 1 =
3. Blower—Mechanically forced ventilation
or recirculation, whether suction or plenum
method; moisture-laden air usually discharged
out of doors.
To understand the particular range of each
of the foregoing types, it is necessary to out-
line the moisture-removal problem in some de-
tail from the standpoint of the lumber to be
dried. Air drying (usually preliminary to kiln
drying) removes more moisture from the sur-
face than from the center, and because of the
length and width of a board, drying takes place
chiefly through the flat faces, rather than
through the ends or edges. The inevitable re-
sult is a surface drier than the interior, and
air-dried stock is therefore subject to an internal
Cut No. 5
*
0
'
A = heating unit;
C = fresh-air
inlet; D and D 1 = used-air outlets;
E = lumber charge; F = humidify-
ing unit
0
i
T
*
0
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charge
The skill of an engineer is not required to
discover that, if the drying of wood afforded
no organic difficulties, the blower would dry
rapidly, utilizing considerable power, and the
Cut No. 7
A = heating onit; C =
•D 1
1. Ventilated—Fresh air taken in and used-
air discharged direct from kiln atmosphere,
problem is to draw the center moisture out and
leave the surface and interior equally dry.
strain that often manifests itself in the form of
warp, twist or other surface irregularities. The
K £ B
E
used-air outlets; E = lumber charge
condenser would function slowly but at a lower
cost. The ventilated kiln is the least under-
stood, even though the most generally used, and
offers an unusual opportunity for speed and
efficiency when its underlying principles are
grasped.
The nature of wood is the phase of the prob-
lem that engineers least appreciate. It is
obvious that wood should not undergo external
or internal strain during drying, but it is prac-
tically impossible to maintain such a "strain-
less" condition. The reduction of this drying
strain to a minimum point is necessary to dry-
ing without damaging the lumber, the usual
manifestations of which are checking, warpmg,
honeycombing, hollow-horning, etc. The chance
of internal strain greatly increases with the
thickness of the wood to be dried; e. g., one-
sixteenth-inch veneer is practically all surface
and can develop little internal strain in any kind
of drying, while four-inch green maple presents
a decidedly subborn drying problem, and taxes
the skill of the best operator with an efficiently
controlled kiln.
Blowers and Condensers
These facts lead to the logical conclusion
that where thickness is nominal, as in the case
of veneer, applied shellac, stain, filler, varnish
and glue, the speed of the air movement in a
blower kiln will remove moisture rapidly and
cause no serious damage; but where thickness
becomes appreciable (one-half inch and up), the
rapidly moving atmosphere of a blower kiln will
inevitably produce uneven drying and unneces-
sary interstrain that will damage the lumber.
Even with attempted maintenance of high hu-
midities (difficult in blower kilns) the hazard
is serious. On the other hand, the slowness
of the condensing kiln will make for accurate
(Continued on page 16)
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